FIG. 1 illustrates a known (prior art) building roof structure R comprising a corrugated metal or other roof deck D supported on plurality of parallel, spaced-apart purlins, trusses or other structural members P that extend axially along respective longitudinal axes PX that each extend perpendicularly into and out of the page as shown in FIG. 1. Between each successive pair of purlins P, an open bay BA is defined, and the roof deck D spans the bays BA. It is known to insulate the roof deck D with a known roof insulation system 10. Typically, as shown in FIG. 1, the known roof insulation system 10 comprises a vapor retarder facing or web or sheet S draped over the respective upper flanges or edges PU of multiple (at least two) successive purlins P so that the sheet S spans and drapes across one or more bays BA. The vapor retarder sheet S is fixed to at least some or all of the purlins P over which it is draped using suitable fasteners, adhesive or other means. The vapor retarder sheet S is a single layer or multiple layer product, e.g., single-layer vinyl film/sheet or other film/sheet, or a laminated composite containing various combinations of aluminum foil, polymeric film/sheet, kraft paper, reinforcing yarns and/or fabrics. Vapor retarder sheets S vary in strength, color, light reflectivity, and their ability to retard moisture migration therethrough. An insulation space SP is thus defined between the inner face S1 of the sheet S and the roof deck D, and fiber glass or other insulation I is laid or blown or otherwise installed in the insulation space SP and is supported on the inner face S1 of the vapor retarder sheet S and/or laminated to the inner face S1 vapor retarder sheet S. The vapor retarder sheet S supports and faces the insulation I, inhibits migration of moisture into the insulation and improves aesthetics of the interior of the building.
The roof insulation system 10 of FIG. 1 has been found to be suboptimal in certain cases. For example, the draped vapor retarder sheet S limits the amount of insulation that can be installed in the bay BA, and the insulation is uneven between the central portion of the bay and the edges adjacent the purlins P. The sheet S can be stretched too taught across the bay BA, which shortens the insulation space SP. As such, the R-value achievable with the system 10 of FIG. 1 can sometimes be limited and/or inconsistent. Also, some people find the draped facing sheet S to lack the desired aesthetics for a roof structure due to the exposed purlins and uneven appearance.